Why has Boeing used the number seven to prefix all of its commercial airplanes? Is there any story behind
the naming of the first model the 707? What does the number signify, if anything?- question from Rajeev Kaul

The 7-7 designation that has come to signify Boeing's line of commercial passenger jets was chosen purely for
marketing purposes, but the story of the 707 is an interesting one,
nonetheless. Boeing, like most other aircraft manufacturers, has used an internal numbering system to identify its
various designs since the company was founded in 1917. The company's first design was the B&W biplane, named after
co-designers William Boeing and G. C. Westervelt. The aircraft was also referred to as the Boeing Model 1.

Boeing Model 1

During the 1930s and 1940s, the company model numbers were well into the 200s and 300s. The famous B-17 Flying
Fortress was known internally as the Boeing Model 299. Other well-known designs of the period included the Model
314 Clipper, the flying boat that ushered in the age of airline travel, and the Model 345, better known as the B-29
Superfortress.

By the start of the jet age, Boeing had progressed to the 400s. Some of its better known designs included the
B-47 Stratojet (Model 450) and
B-52 Stratofortress (Model 464). These two aircraft made Boeing the
nationally-recognized leader in the design and construction of large, jet-powered aircraft. This wealth of
experience would prove vital in producing the country's first commercial jetliner.

B-47 Stratojet, Boeing Model 450

The effort began when Boeing initiated a study of a turbojet-powered version of the KC-97 Stratofreighter. Known
internally as the Model 367, the KC-97 was a piston-engined derivative of the B-29 developed as an in-flight
refueling tanker for the B-47 and B-52. The same design had also been adapted as the Model 377 Stratocruiser
commercial airliner. Although Boeing was eager to develop a jet-powered airliner to replace the Model 377, the
company knew that airlines would not commit to the enormous development costs required. It was instead hoped that
the US Air Force would fund development of the jet-powered variant of the KC-97, and the final product could then
be adapted into a civilian spin-off.

KC-97 refueling a B-47

Unfortunately, the Air Force showed no interest in the proposal, and it was rejected in 1951. Boeing instead
decided to commit over $15 million of company funds to develop the turbojet-powered transport, convinced that
the USAF would eventually buy it anyway. Owing to its legacy as a derivative of the Model 367 and in an effort to
maintain secrecy, the new project was dubbed the Model 367-80, popularly known as the "Dash-80" by Boeing
employees. However, the Dash-80 soon evolved far beyond its initial concept. The design ultimately became much
larger, with a longer, wider, and more streamlined fuselage able to hold 130 passengers. The overall "figure-8"
fuselage shape was derived from the KC-97 and Stratoliner, but faired over to improve aerodynamics. Otherwise, the
design was largely inspired by the B-47, having borrowed the bomber's 35° swept-wing and the mounting of its
turbojet engines on underwing pylons.

Boeing Model 367-80, prototype of the 707

By the time the Dash-80 made its first flight in 1954, the Air Force had changed its mind about the project and
soon ordered 29 as KC-135 tanker/transports. Airline interest was
somewhat slow in coming, however, so Boeing officials looked to give the project a new, more memorable name. The
Model 367-80 had never been intended as anything more than an internal designation. Models in the 500s had been
reserved for gas turbine engines and the 600 series for missiles. The 700 series was again intended for aircraft,
so the company board decided to officially christen the Dash-80 as the Model 707.

Indeed, many of Boeing's most successful products over the years had had a connection to the number 7. These
included the Model 75--hundreds of which were used as trainers during World War II, the Model 77--one of the first
carrier-based fighters, the Model 247--considered to be the prototype of the modern airliner, the B-17 Flying
Fortress--of which 13,000 were built for use during World War II, and the Model 314 Clipper--greatest of the
passenger flying boats. Other notable aircraft included the B-29 Superfortress (i.e. 9-2=7) that saw great success
in the Pacific Theater of World War II and was developed into a number of derivatives, including the Model 367
and Model 377, as well as the B-47 Stratojet and B-52 Stratofortress (5+2=7).

With over 1,000 built by the time production closed in 1991, the 707 proved such a huge success that Boeing
decided to retain the same naming convention for nearly all of its future jetliners. One exception was the
Model 717 which was originally given to the military version of the 707, better known as the KC-135. However,
that designation did not last and was later briefly given to a different commercial derivative of the 707. Even
later, it was applied to the MD-95 after Boeing had acquired its former rival McDonnell Douglas. As the only
McDonnell Douglas production line to remain open for long after the merger with Boeing, the MD-95 was re-christened
the 717 in 1997. The only other unusual case was the Model 720, a shorter range version of the 707 developed for
domestic use. The 720 began life as the 707-020 and was later briefly known as the 717-020 before the Model
720 became official.

Otherwise, Boeing has retained the 7-7 convention for the past fifty years, having applied it to its entire family
of commercial jetliners:

The latest member of the family is the 787, formerly known as the 7E7
because of its use of "super-efficient" technologies.

As for the plane that started it all, the Dash-80 still holds an honored place in aviation history. The
prototype was a workhorse for 18 years, serving as a Boeing flying test laboratory for new technologies. Over the
course of its career, the aircraft was heavily modified to test aerodynamic and structural changes as well as
advanced features that were incorporated into later planes. Among these modifications were new wing planform and
leading edge shapes, airfoil designs, leading-edge and trailing-edge wing flap concepts, and new engines. The
Dash-80 even carried a fifth engine in an aft-mounted pod to evaluate a concept for the 727. The historic aircraft
was finally retired in 1972 when it was donated to the National Air & Space Museum. However, the Dash-80 remained
stored at the Davis-Monthan Air Force Base desert storage facility until
1990. At that time, the Smithsonian agreed to return the prototype to Boeing's facilities in Seattle for a full
restoration to its original form. There the Model 367-80 has remained, being displayed at Boeing Field. It is due
to be transferred to the National Air & Space Museum
Steven F. Udvar-Hazy Center in the fall of 2003.
- answer by Joe Yoon, 27 July 2003